1. Field of the Invention
The present invention relates to a method and an apparatus for stacking sheets and, more particularly, to a method and an apparatus for stacking a pair of glass substrates constituting a liquid crystal display panel.
2. Discussion of Related Art
There has been a conspicuous rise in the popularity of a liquid crystal display device used as an image display device for a personal computer and various other monitors. In general, such a liquid crystal display device is constituted so as to make an image formed on a liquid crystal surface visible by disposing a backlight as a sheet-like light source for lighting in the backside of a liquid crystal display panel and by lighting the entire liquid crystal surface having a specified width to uniform brightness. This liquid crystal display device includes a liquid crystal display panel having liquid crystal sealed in between a pair of glass substrates.
The liquid crystal display panel is manufactured mainly by a process called a vacuum injection method. According to the vacuum injection method, a pair of glass substrates having a predetermined distance therebetween are arranged oppositely to each other, a panel including a liquid crystal inlet is dipped in liquid crystal after the execution of degassing between the glass substrates under vacuum, then the atmospheric pressure is restored and, by using a pressure difference between the inside and the outside of the panel and capillarity, the liquid crystal is injected into the panel. The panel can be obtained by applying sealant around the pair of glass substrates in a picture-frame state, and then curing the sealant. A region surrounded with the sealant of the picture-frame state becomes an image display region, and the liquid crystal is injected through the liquid crystal inlet into this image display region. The liquid crystal inlet is sealed off after the liquid crystal is injected. For a sealant material, a thermosetting or ultraviolet curable resin is used, and the sealant is cured before injection of the liquid crystal.
A problem inherent in the vacuum injection method has been long time expended for injection of the liquid crystal. In particular, there has been a problem that a great deal of time required for injection when the size of the liquid crystal display panel becomes larger.
In addition to the above-described vacuum injection method, a liquid crystal sealing-in process called a dropping method is available. According to the dropping method, sealant is coated in a picture-frame state around one of the pair of glass substrates, the other glass substrate is stacked after liquid crystal is dropped on a region surrounded with the sealant, and then the sealant is cured. Compared with the vacuum injection method, this dropping method is advantageous in that time required for liquid crystal dropping is greatly shortened. Therefore, considering manufacturing costs, the dropping method is a good manufacturing method.
Regarding the stacking of the glass substrate after the dropping of liquid crystal, the dropping method has a method executed in the atmosphere, and a method executed in a vacuum device. The method of stacking the glass substrate in the atmosphere is advantageous in that manufacturing costs can be reduced by an amount equivalent to the nonuse of the vacuum device. However, bubbles tend to remain on a stacking surface during the stacking of the glass substrate in the atmosphere. These bubbles may lead to a display failure in the liquid crystal display device. Many suggestions have been presented to solve this problem of residual bubbles. For example, Japanese Patent Laid-Open No. Hei 4 (1992)-179919 discloses a stacking method of widely spreading liquid crystal on the full surface of the glass substrate while narrowing an angle between the pair of glass substrates, liquid crystal having been coated on the corner of one glass substrate at the point of intersection where the pair of glass substrates are arranged to face each other in a surface direction in a wedge shape. Also, Japanese Patent Laid-Open No. Hei 4(1992)-179919 or Japanese Patent Laid-Open No. 2000-29051 discloses a method of widely spreading liquid crystal on a full surface while returning the glass substrate to its original planar state after the pair of glass substrates are placed to face each other, and liquid crystal is interposed while at least one glass substrate is bent in a projecting shape.
The above-described methods reduce residual bubbles. However, as long as stacking is carried out in the atmosphere, bubbles always remain to a slight extent. Especially when higher image quality is required, these methods cannot provide sufficient countermeasures.
It is therefore considered preferable to execute the stacking of the glass substrate in vacuum even in the dropping method. However, for stacking the glass substrate in vacuum, there is a problem regarding how to hold the glass substrate in vacuum. In other words, to stack the pair of glass substrates., at least one glass substrate must be aligned with the other glass substrate while it is being held. But no proper means for holding the glass substrate has been discovered yet.
As one of the most general glass substrate holding methods in the atmosphere, a vacuum chucking or holding method is available. To begin with, however, the vacuum chucking method cannot be used in vacuum, where provides no differential pressures. An electrostatic chucking or holding method using static electricity enables to hold the glass substrate in vacuum. However, in the electrostatic chucking method, it takes a long time-until securely holding. In addition, an electric circuit is provided on the glass substrate of the liquid crystal display device, and there is a danger that the electrostatic chucking may cause electrostatic destruction in the circuit.
If a mechanical holding method is used, the glass substrate can be held in vacuum, holding time can be short, and the electrostatic destruction of the circuit can be prevented.
The glass substrate used for the liquid crystal display device is thin, having a thickness set equal to about 0.7 mm, and a distance between the pair of glass substrates is very small, that is, 10 micrometers or less. Further, the pair of glass substrates must be stacked in the state of highly accurate alignment. Accordingly, even if the mechanical holding method is employed, a method and an apparatus for stacking should be selected with consideration given to the above point.
The present invention was made with the foregoing problems in mind, and the object of the invention is to provide an apparatus and a method for stacking sheets accurately even in vacuum. Another object of the invention is to provide a method and an apparatus for manufacturing a liquid crystal display panel by using such stacking apparatus and method.